The service brake of a motor vehicle is typically actuated when a driver of the vehicle uses his/her foot to depress a brake pedal. In a hydraulic brake system, depression of the pedal acts through a pushrod connected to a master cylinder to cause hydraulic brake fluid that is trapped in brake lines between the master cylinder and hydraulic actuators at the wheels to be displaced in a manner that moves either a shoe in the case of a shoe brake or calipers in the case of a disc brake to frictionally engage either a drum or a rotor that turns with the wheel. In an air brake system, the pedal still acts through a pushrod, but on an air brake valve instead of on a master cylinder.
In some production vehicles, the brake pedal is suspended from a brake pedal mounting bracket for swinging motion about a pivot axis. In a representative vehicle, a brake pedal tube, to which one end of a brake pedal arm is joined, is disposed to turn on a horizontal pedal mounting shaft that is supported on vertical side walls of the bracket and that defines the pivot axis of the brake pedal. A pushrod is connected by a suitable connection, such as a clevis, to the pedal arm at some distance below the pivot axis. A pedal pad that is pushed by a person's foot to apply the service brake is at an end of the pedal arm below the point of connection of the pushrod to the pedal arm.
From its connection to the pedal arm, the pushrod extends in a sealed manner through a hole in a front wall of the bracket and an aligned hole in the vehicle dash panel to a master cylinder in the case of a hydraulic brake system or to an air brake valve in the case of an air brake system. A return spring acts on the pedal arm to return it to its initial position when a person's foot that had been depressing the pedal pad is removed from the pedal pad.
The geometry of this representative arrangement provides a mechanical advantage based on the ratio of the distance of the pedal pad from the pivot axis to the distance of the connection of the pushrod to the pedal arm from the pivot axis. The mechanical advantage, or pedal ratio, is one factor in the feel that the brake system presents to a driver's foot pressing on the pedal pad to apply the service brake.
Establishing an acceptable feel for the pedal is part of the brake system development process. In conjunction with inputs from any of various sources, such as product planners, user surveys, competitive product evaluations, etc., brake system designers and engineers are tasked with creating a system having pedal feel that is considered acceptable to as large a population of drivers as reasonably practical.
If a system is designed and engineered with the expectation that it will provide an acceptable pedal feel to most drivers, but when constructed and tested during the prototype phase is found not to do so, a typical practice has been to design and fabricate new parts that will provide a different pedal ratio hopefully providing the desired result.
The new parts require time to fabricate. An existing prototype brake system must be disassembled to remove parts that are to be replaced before the new parts can be installed. This process may have to be repeated multiple times. Consequently, valuable time may be lost in the development process, and added development costs may be incurred.